In a typical wireless system such as WCDMA, the baseband signal processing must meet two main specifications: (1) the in-band Error Vector Magnitude (EVM), and (2) the out-of-band Adjacent Channel Leakage Ratio (ACLR) as well as other speicifcations. The limits imposed on both EVM and ACLR are stringent.
For WCDMA applications, there is an in-band EVM specification for the transmitter and an out-of-band ACLR specification at 5 MHZ and 10 MHZ. Usually, there is strong filtering at 10 MHZ which helps in reducing the 10 MHZ ACLR specification, but 5 MHZ is so close to the signal band that the 5 MHZ ACLR specification is somewhat more difficult to meet. Because all of the blocks in the transmit chain have an effect on meeting the ACLR specification, the effect of each should be well below the specification for the overall specification of the system to be met.
The ACLR specification at 5 MHZ determines the number of bits required in the digital-to-analog converter. For WCDMA applications, this number is usually nine or ten bits. The other specifications, namely EVR and the 10 MHZ ACLR) usually can be satisfied with six bits.